1. Field of the Invention
The present invention relates to wellbore surveying systems and techniques. More particularly, the present invention relates to systems techniques for surveying wellbores and/or determining position of a wellbore in the Earth.
2. Background of the Related Art
Wellbores are drilled to locate and produce hydrocarbons. A downhole drilling tool with a bit at an end thereof is advanced into the ground to form a wellbore. As the drilling tool is advanced, a drilling mud is pumped from a surface mud pit, through the drilling tool and out the drill bit to cool the drilling tool and carry away cuttings. The fluid exits the drill bit and flows back up to the surface for recirculation through the tool. The drilling mud is also used to form a mudcake to line the wellbore.
Fluids, such as oil, gas and water, are commonly recovered from subterranean formations below the earth's surface. Drilling rigs at the surface are often used to drill wellbores into the Earth's crust to the location of the subsurface fluid deposits to establish fluid communication with the surface through the drilled wellbore. In many cases, the subsurface fluid deposits are not located directly below the drilling rig surface location. In these cases, a “directional wellbore” is drilled. A directional wellbore is a wellbore that deviates from vertical. Downhole drilling equipment may be used to directionally steer the drilling tool to drill the wellbore to known or suspected fluid deposits using directional drilling techniques to laterally displace the borehole and create a directional wellbore.
Directional wellbores are drilled through Earth formations according to a selected or desired trajectory, however, many factors may combine to unpredictably influence the actual trajectory of a wellbore. It is desirable to accurately determine the wellbore trajectory in order to guide the wellbore to its geological and/or positional objective. Thus, it may be desirable to measure the inclination, azimuth, depth, and position of the drill bit during wellbore operations to determine whether the selected trajectory is being maintained within acceptable limits.
Surveying of wellbores is commonly performed using downhole survey instruments. These instruments typically contain sets of orthogonal accelerometers, magnetometers, and/or gyroscopes. These survey instruments are used to measure the direction and magnitude of the local gravitational field, magnetic field, and Earth spin rate vectors. These measurements correspond to the instrument position and orientation in the wellbore, with respect to these vectors. Wellbore position, inclination, and/or azimuth may be estimated from the instrument's measurements. Techniques for surveying of wellbores are disclosed in U.S. Pat. No. 5,452,518 to Dispersio; U.S. Pat. No. 5,606,124 to Doyle, et al.; GB Patent No. 2351807A to Shirasaka, et al.; U.S. Pat. No. 5,657,547 to Uttecht, et al.; and Patent Publication No. 2004/0107590 A1 to Russell, et al.
In general, wellbore surveys are performed by while-drilling tools that are located in the bottom hole assembly (“BHA”) of a drilling system. One technique is to wait for a break in the drilling process, which typically happens when additional sessions of drill pipe are being added to the drill string. When the drilling has stopped, the survey instruments may make measurements that are not affected by the movement and vibrations that are created by the rotation of the drill string and the action of the drill bit on the bottom of the hole. It is noted that this is only one example of a technique for making wellbore surveys. Wellbore surveys may be initiated and acquired at any time, including during drilling operations. In addition, wellbore surveys may be performed by wireline tools that are run into the wellbore when the drill string has been removed or that are run inside the drill string.
There are many sources of measurement uncertainty and inaccuracy. For example, magnetic measuring techniques suffer from the inherent uncertainty in global magnetic models used to estimate declination at a specific site, as well as local perturbations in the magnetic field due to the nearby magnetic materials or the casing of the wellbore or of a nearby well. Similarly, gravitational measuring techniques suffer from movement of the downhole tool and uncertainties in the accelerometers. Gyroscopic measuring techniques, for example, suffer from drift uncertainty. Depth measurements are also prone to uncertainties including mechanical stretch from gravitational forces and thermal expansion, for example.